Power assist steering systems may include a rack and pinion gear set. Power assist is accomplished by applying a supplementary rotary force to a steering member. Such systems may be controlled in response to a driver's applied torque to a vehicle's steering wheel.
U.S. Pat. No. 4,660,671 utilizes a DC electric assist motor. The motor includes a rotatable armature encircling a steering member which has a thread convolution portion thereon and a portion having straight cut rack teeth thereon. Rotation of the electric assist motor armature causes linear movement of the steering member to a ball-nut drive arrangement in combination with the thread convolution portion of the steering member. A torque sensing device is coupled to the steering column to sense driver applied input torque to the steering wheel. The torque sensing device uses a magnetic Hall-effect sensor arrangement for sensing relative rotation between the input and output shafts across a torsion bar. An electric control circuit monitors the signal from the torque sensing device and controls the electric assist motor in response thereto.
U.S. Pat. No. 4,682,104 discloses an angular displacement sensor for use in detecting torque in a power assist steering system. The torque sensor includes an input planar rotary transformer which includes a stationary primary winding and a rotary secondary winding. The secondary winding is mounted on one face of an annular disc which is, in turn, mounted on the input shaft for rotation therewith. Axially displaced from the rotary transformer there is provided a pair of sensor or detection discs including a transmitter disc and a receiver disc. The opposing faces of these discs carry identical planar coils with the coils being in the shape of spirally wound sectors coiled alternately in opposing directions and coaxially surrounding the common axis of the input and output shafts. An AC voltage supplied to the primary of the rotary transformer is applied to the planar coils on the transmitter disc. This causes a voltage to be induced in the planar coils mounted on the receiver disc. A single output planar rotary transformer is employed having a primary winding mounted on a disc and coaxially surrounding the output shaft. The output transformer includes a single stationary winding on another disc axially spaced from the primary winding. A single output signal is provided which varies with angular deviation between the input and output shafts.
A problem noted with respect to the torque sensor disclosed in the aforesaid U.S. Pat. No. 4,642,104 is that the output transformer provides a single output signal representative of the angular displacement between the input and output shafts. This is an absolute value signal and is subject to providing erroneous information. Preferably, two output circuits should be employed providing two output signals that are ratiometric wherein as one signal increases in amplitude, the other decreases in amplitude in an equal amount providing a stable null signal condition. However, such a two-output circuit is not practical for a device constructed in accordance with the aforementioned patent which employs multiple planar discs for both the input rotary transformer and the single output rotary transformer. To add an additional output rotary transformer would present practical problems of planar disc spacing tolerances which would limit axial movement of the discs relative to each other.